The present invention relates to a burst signal generator which is used for supplying a so-called burst signal of a certain period to a device under test, for example, for analyzing its frequency characteristics by a digital spectrum analyzer.
A conventional burst signal generator is designed so that an oscillator of a constant frequency is switched ON and OFF by a FET or like switching means and the oscillator output is supplied to, for example, a device under test during the ON period alone, or the oscillator output is amplitude modulated 100% by a rectangular wave signal of a fixed frequency, that is, intermittently modulated, thereby obtaining a burst signal as the modulated output.
With such a prior art burst signal generator, however, it is difficult to accurately obtain a preset number of waves during the period of the burst signal. Heretofore there has not been proposed such an arrangement that start and end phases of the burst signal are preset so that the burst signal starts and ends with the preset start and end phases, relatively. For example, in the case of measuring the time response of an automatic control system, or in measuring the response of a device when it is mechanically or electrically vibrated, the response characteristic sometimes considerably differs with the value of the end phase of the applied mechanical or electrical burst signal. In such a case, it is desired to produce a burst signal which stops with a preset end phase (stop phase). Further, it is sometimes desired to measure a response to the application of two consecutive burst signals. For producing the two burst signals in succession, it is necessary to set the start phase of the succeeding burst signal in such a manner as to immediately follow the end phases of the preceding burst signal. In the past, however, there has not been proposed an arrangement which produces a burst signal with a start phase of such a preset value.
In Japanese Patent Application No. 167370/82 (U.S. patent application Ser. No. 533,780 filed on Sept. 19, 1983) which had not been published before the filing of the basic Japanese patent application of this application, it is disclosed to prestore waveform data (amplitude values) of a signal of one cycle in a waveform memory at fixed phase intervals, to read out the waveform memory at a constant rate and to convert the read-out waveform data into an analog signal. Further, it is disclosed to raise the frequency of the output signal by reading out the waveform memory while skipping over a fixed number of addresses for every readout clock and to reduce the frequency of the output signal by reading out the waveform memory at one address for every plurality of readout clocks. This prior application, however, does not describe to read out the waveform memory by a preset number of waves and to start and stop the generation of the output signal with preset phases, respectively.